电子态调控促进氢气无损耗纯化中CO的光致富集和氧化

doi:10.1016/S1872-2067(23)64483-3

催化学报 ›› 2023, Vol. 51: 49-54.DOI: 10.1016/S1872-2067(23)64483-3

电子态调控促进氢气无损耗纯化中CO的光致富集和氧化

刘海峰^a^,^c, 黄祥^b^,^*(), 陈加藏^a^,^c^,^*()

^a中国科学院山西煤炭化学研究所煤炭高效低碳利用全国重点实验室, 山西太原030001
^b南方科技大学物理系, 广东深圳518055
^c中国科学院大学材料与光电研究中心, 北京100049

收稿日期:2023-05-27 接受日期:2023-06-28 出版日期:2023-08-18 发布日期:2023-09-11
通讯作者: *电子信箱: huangx8@sustech.edu.cn (黄祥), chenjiazang@sxicc.ac.cn (陈加藏).
基金资助:
国家自然科学基金(22172185);国家自然科学基金(21773285);中国科学院西部青年学者(XAB2019AW09)

Surface electronic state modulation promotes photoinduced aggregation and oxidation of trace CO for lossless purification of H₂ stream

Haifeng Liu^a^,^c, Xiang Huang^b^,^*(), Jiazang Chen^a^,^c^,^*()

^aState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Shanxi, China
^bDepartment of Physics, Southern University of Science and Technology, Shenzhen 518055, Guangdong, China
^cCenter of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China

Received:2023-05-27 Accepted:2023-06-28 Online:2023-08-18 Published:2023-09-11
Contact: *E-mail: huangx8@sustech.edu.cn (X. Huang), chenjiazang@sxicc.ac.cn (J. Chen).
Supported by:
National Natural Science Foundation of China(22172185);National Natural Science Foundation of China(21773285);CAS Western Youth Scholars Program(XAB2019AW09)

摘要/Abstract

摘要：

随着选择性氧化(PROX)的快速发展, 将氢气中残留的微量CO去除以达到质子交换膜燃料电池的使用氢要求, 成为氢气纯化中迫切需要解决的问题. 光催化氧化可将氢气流中的CO浓度降低至1 × 10^-6以下. 本质上, 该反应是光生•OH与痕量CO在氢气中反应. 在该过程中, 虽然•OH会不可避免地与H₂发生碰撞而形成氢自由基(•H), 但是环境中的H₂会与之结合并通过氢交换促进自由基间的有效碰撞而结合成H₂, 避免氢气损耗. 相比于现有的氢气纯化方案, 光催化CO氧化可发展成为一种不损耗氢气的纯化技术. 该技术虽然具有应用前景, 但•OH的非选择性碰撞严重降低了其利用率以及整个氢气无损耗纯化反应过程中的光子利用率, 因此, 提出可行方案来促进•OH的产生及其与CO的碰撞成为研究重点.
本文采用密度泛函理论结合Climbing image-nudged elastic band方法研究水分子在TiO₂上的反应过程. 在普通TiO₂(101)晶面上形成^*OH的势垒为0.57 eV; 而采用Mg来取代TiO₂的5c-或者6c-Ti位点后, 势垒分别降至0.25和0.2 eV, 并且^*OH吸附在相邻Ti位点上的自由能比•OH高. 除了促进光致•OH的产生, Mg掺杂所形成的位点还能促进发生在半导体-助催化剂界面秒级别的电子转移, 进一步提高光子利用率. 而且, Mg掺杂降低了TiO₂中Ti⁴⁺的电子云密度, 促进了CO分子中碳5σ轨道上的负电荷转移至半导体阳离子, 同时增强了半导体中阴离子向CO分子转移负电荷的能力, 由此可以增强所形成的Mg掺杂TiO₂(Mg-TiO₂)与CO的静电作用, 实现该气体在半导体附近的富集. 因此, Mg掺杂不仅可以促进羟基自由基的产生, 还可以通过静电作用促进CO在Mg-TiO₂表面的富集, 提高自由基对目标反应的碰撞概率. 在光催化CO氧化反应中, Mg-TiO₂展现出比普通TiO₂更优的性能; 并且随着CO浓度降低, Mg掺杂带来的CO富集效应愈加显著, 进而表现出更好的催化性能. 在光催化氢气无损耗纯化反应中, 基于Mg-TiO₂催化剂的光催化固定床反应可将流速为1 L/min的H₂气流中CO/H₂的比浓度降低至< 1 × 10^-6, 并展现出高效的光子利用. 与其它将CO/H₂比浓度降低至10^-6的方法相比, 本文催化剂气时空速可达600000 h^-1, 远高于PROX的5000 h^-1以及CO甲烷化的7500 h^-1. 综上, 本工作揭示了电子态调控对CO光致富集和氧化的促进作用, 对氢气无损耗纯化的光子利用效率提升和商业化具有借鉴意义.

关键词: 电子态, 羟基自由基, 一氧化碳氧化, 氢气纯化, 光催化固定床反应

Abstract:

Although photogenerated hydroxyl radicals can oxidize CO without H₂ consumption, the nonselective collision severely lowers photocatalytic photon utilization. We here demonstrate that electronic state modification of TiO₂ resulted from magnesium doping can promote hydroxyl radical generation by weakening the adsorption of oxygen species and facilitating semiconductor-cocatalyst interfacial electron transfer. Importantly, the partial deprivation of electronic cloud from cationic sites can strengthen σ-donation and π-backdonation that synergistically promote the electrostatic interaction for aggregating CO in the vicinity of semiconductor. The resulted photocatalyst thus exhibits growing superiority over common counterpart in target reaction as CO concentration lowers. By these merits, CO in H₂ stream can be reduced to < 1 × 10^-6 with efficient photon utilization.

Key words: Electronic states, Hydroxyl radical, Carbon monoxide oxidation, Hydrogen purification, Photocatalytic fixed bed reaction

刘海峰, 黄祥, 陈加藏. 电子态调控促进氢气无损耗纯化中CO的光致富集和氧化[J]. 催化学报, 2023, 51: 49-54.

Haifeng Liu, Xiang Huang, Jiazang Chen. Surface electronic state modulation promotes photoinduced aggregation and oxidation of trace CO for lossless purification of H₂ stream[J]. Chinese Journal of Catalysis, 2023, 51: 49-54.

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Fig. 1. (a-c) Comparison of minimum energy pathways for H2O dissociation on TiO2 (101) surface with and without Mg doping. (d) Adsorption free energies of *OH adsorbed on the Ti sites on perfect TiO2 (101), 5c@Mg, and 6c@Mg. The dashed line indicates the free energy of a free solvated ?OH. The light blue, green, red, and white balls represent Ti, Mg, O, and H atoms, respectively.

Fig. 2. X-ray diffraction patterns (a), electron microscopy images (b,c), high-angle annular dark field (haadf) and elementary mapping images (d), UV-vis absorption spectra (e), and Ti 2p (f), O 1s (g), and Mg 1s (h) X-ray photoelectron spectra of TiO2 (b) and Mg-TiO2 (c,d,h). Although the phase (a), size (b,c), and light absorption behaviors (e) do not change, together with the elementary analysis (d,h), the blueshift of Ti 2p peaks (f) and the redshift of O 1s peak (g) suggest that modification of electronic states occurs after magnesium doping. The length of scale bars shown in haadf and elementary mapping images is 100 nm (d).

Fig. 3. Photoinduced anodic behaviors of TiO2 and Mg-TiO2 on the glassy carbon disk and the related cathodic current occurring on the platinum ring of a rotating ring-disk electrode. The ring is biased at 0 V (vs. Ag/AgCl), which is more positive than the potentials for hydrogen evolution in the solution (1 mol/L Na2SO4). The cathodic signal can reflect the reduction of hydroxyl radicals generated on the disk electrode.

Fig. 4. Photoinduced generation of hydroxyl radicals over TiO2 and Mg-TiO2 in 2 mmol/L NaOH solution containing 0.5 mmol/L terephthalic acid (TA) trapping agent. The generation of hydroxyl radicals can be partially reflected by the formation of 2-hydroxyterephthalic acid (HTA), which formed by reacting TA with the captured hydroxyl radicals. The formation rates of HTA estimated from the fluorescence spectra (a,b) are respectively 0.129 and 0.230 μmol/h for TiO2 and Mg-TiO2 (c). Since the concentration of HTA is far lower than that of TA even after reaction (60 min), it can safely conclude that the formation rate of hydroxyl radicals over Mg-TiO2 is 0.78 times higher than that of TiO2.

Fig. 5. Time constants for leakage of electrons from TiO2 and Mg-TiO2 to the solution and for interfacial transfer of electrons from TiO2 and Mg-TiO2 to copper (a). The percentage of electrons transferred to copper (b) can be estimated from time constants (a), which can be obtained by monitoring the open-circuit potential decay from photoinduced potential (Fig. S2).

Fig. 6. Photoinduced CO oxidation over TiO2|Cu and Mg-TiO2|Cu photocatalyst. CO2 generation rate (a) and the rate ratio (b) can be evaluated by monitoring the effluent gas (Fig. S5). The concentration of CO in the feeding stream (30 mL/min) is 100 vol%, 10 vol%, 1 vol%, and 0.1 vol%.

Fig. 7. The infrared absorption behaviors of CO over TiO2 and Mg-TiO2 in dark and under irradiation.

Fig. 8. Photoinduced oxidation of CO (10 × 10-6) in H2 stream (1 L/min). With increase in the number of reaction chambers (a), the concentration of CO in the effluent H2 stream decreases (b). According to the simulated CO concentration (solid line), the predicted behavior (extended dish line) suggests that TiO2|Cu photocatalyst needs at least 4 reaction chambers (open squares) to reduce CO to < 1 × 10-6 in the effluent H2 stream (b). The long-term operation shows that by connecting two reaction chambers, the Mg-TiO2|Cu photocatalyst can reduce the concentration of CO to < 1 × 10-6 (0.7-0.9 × 10-6) in effluent H2 stream after 1 h of initialization (c). For the case of TiO2|Cu photocatalyst, CO in the effluent gas is ~4 × 10-6 (c). In each reaction chamber, the irradiation (UVC, 254 nm) provided by a 320-W amalgam lamp is 105 W (a). The moisture (~10000 × 10-6 H2O) is provided by injecting water, which can be readily vaporized in the reaction chamber that kept at 90 °C. Compared with the highest level of PROX [24], methanation [25], and photocatalytic radical reaction [4], our work highlights an important strategy for hydrogen purification (d,e). The schematic diagram (a) is adapted from ref [4]. Copyright 2022, American Chemical Society

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电子态调控促进氢气无损耗纯化中CO的光致富集和氧化

Surface electronic state modulation promotes photoinduced aggregation and oxidation of trace CO for lossless purification of H₂ stream

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电子态调控促进氢气无损耗纯化中CO的光致富集和氧化

Surface electronic state modulation promotes photoinduced aggregation and oxidation of trace CO for lossless purification of H2 stream

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Surface electronic state modulation promotes photoinduced aggregation and oxidation of trace CO for lossless purification of H₂ stream